A highly basic granular flux for submerged arc welding is normally classified as H8 or H16 (as per AWS A4.3-93) when used with various AWS classes of electrodes. Consequently, the existing granular flux for submerged arc welding results in relatively high diffusible hydrogen in the weld metal. It is a well documented fact in the technical literature that a high diffusible hydrogen level makes the weld metal more prone to hydrogen assisted cold cracking (HACC). Achieving a low weld metal diffusible hydrogen level is now more critical due to the recent trend toward higher strength steels. Also, it is a known fact that a low nitrogen level in the weld metal enhances the weld mechanical properties, especially the weld metal toughness. Given the applications they are used in, having high toughness levels is critical for highly basic flux. Good slag removal is also important for highly basic fluxes, especially in narrow deep grooves of the type used in off shore welding, ship building, pressure vessel and wind tower constructions. In these structural fabrication environments, a narrow groove is provided in heavy metal workpieces with a thickness substantially greater than about 0.5 inches. With a narrow groove, as used on the European continent and relatively tight grooves, as used elsewhere, the first pass of a submerged arc welding process involves depositing a substantial amount of molten metal in a very deep, narrow groove. Thus, it is a disadvantage to have a flux that creates a slag that does not easily release from the molten metal bead and a bead with high diffusible hydrogen and inclusions of nitrogen. With all of these strict requirements for a deep groove weld bead, the flux is an essential element to be controlled in the combination of granular flux with any submerged arc welding electrode. The flux must be formulated and physically designed to produce low hydrogen, low nitrogen and acceptable slag removal. This type of flux must be capable of use with a wide variety of various electrodes without deteriorating the physical characteristic of the weld metal or the removability of the covering slag. All of these requirements of a submerged arc flux for use in deep narrow grooves have resulted in very high cost flux even though this costly flux is not necessary during the subsequent welding passes. Thus, there is a need for a low cost, granular flux to be used with a variety of electrodes in a deep groove submerged arc welding process which will result in quality weld metal as well as excellent slag removability in the toe crevices of the resulting bead. Such flux can therefore be used for deep groove submerged arc welding and is still economical for the filler passes of the groove.
There is provided a highly basic submerged arc flux having a basicity index B.I. greater than 2.0 and preferably about 2.6 per the Boniszewski formula:
      B    .    I    .    =                    0.5        ⁢                  (                                    Fe              ⁢              O                        +            MnO                    )                    +      CaO      +      MgO      +                        Na          2                ⁢        O            +                        K          2                ⁢        O            +              CaF        2                            SiO        2            +              0.5        ⁢                  (                                    TiO              2                        +                          ZrO              2                        +                                                            (                                      A                    ⁢                    l                                    )                                2                            ⁢                              O                3                                              )                    
This highly basic submerged arc flux, in accordance with the present invention, generates less than 4 ml/100 g of diffusible hydrogen in the weld metal deposit. Thus, the highly basic submerged arc flux of the present invention is classified as an H4 flux (as per AWS A 4.3-93) with a diffusible hydrogen level less than 4.0. The flux is primarily used for the bottom weld of a deep groove. In the past, the basic submerged arc fluxes used economically in the deep groove environment had a basicity index of less than 2.0 calculated using the Boniszewski formula. The highly basic fluxes with a basicity index >2.0 that were used in the deep groove environment were either expensive or it was costly to remove the slag. By formulating the submerged arc granular flux as provided by the invention, a low cost highly basic flux is obtained with a classification of H4. This flux is compared to the prior highly basic fluxes having a classification of H8, or above. Thus, the present invention is the first highly basic H4 granulated flux for use in submerged arc that is not formulated by high priced ingredients or techniques making the flux economical in only the most restricted submerged arc welding operations. In the field, with multiple passes, the operator uses only one flux. In the past this need for a single flux was a costly necessity.
The present invention employs a low cost H4 classified granular flux which can be used successfully in the bottom of a narrow deep gap and that is also cost effective for use in subsequent submerged arc passes of a deep groove. There is no need for a special high cost submerged arc granular flux to be used in the deep bead while employing a less expensive granular flux in the upper weld passes filling a deep narrow groove. The present invention utilizes low cost constituents and is processed in accordance with standard technology to produce a weld metal that has low diffusible hydrogen and excellent slag removal even at the bottom of a narrow deep groove. The narrow deep groove used mainly in Europe, but also in other places for off shore welding and other high strength structural welding employs an extremely narrow groove to reduce the cost of the weld. This narrow groove requires a good slag removal, especially on the first pass. It is difficult to remove the slag from the bottom of a deep groove, when the groove is extremely narrow. A less restrictive, but still challenging narrow groove configuration has a wall angle of about 30°; however, this is still a type of deep groove submerged arc welding with the requirement of good slag removal and low diffusible hydrogen obtained by the present invention. The slag from these deep narrow grooves is often removed by a hand operation; therefore, resistance to removal must be slight, if any. Furthermore, the slag residue must not penetrate into the toe between the first pass and the sidewalls of the narrow, deep groove.
The present invention provides a weld in the lower area of a narrow, deep groove which weld is essentially clean with no residue. Since the cost of the granular flux of the present invention is generally commensurate with normal submerged arc flux, the novel flux can be used economically in all passes in the narrow groove. There is no technical or economical reason to use different fluxes for the various welding passes in the deep groove.
The novel flux is used for double ending pipe sections to produce quality welds preparatory to the joined sections being removed from the welding shop for pipeline construction in the field. The flux is used in off shore welding, structural fabrication, pressure vessels, ship building and wind towers. Its use results in low diffusible hydrogen levels in the weld metal, which hydrogen level is defined as being under 4 ml/100 grams of weld metal. This novel flux is designed to deliver easy deep groove slag removal and yields excellent weld metal toughness. The new flux provides hydrogen levels of less than H4 classification as per AWS A4.3-93. Tests indicate easier deep groove slag removal than other fluxes in this basicity. The basicity index for the preferred embodiment is 2.6 and the particle size is generally between 10 mesh and 60 mesh, indicating that the particle size is in the general range of 250-2,000 microns. This particle size produces a packing density or “apparent density” of the flux pile that optimizes the flux burden. The flux burden is well balanced between allowing gaseous removal and covering the molten metal weld bead prior to solidification. The recommended electrode for use in submerged arc welding with the new flux for mild steel under AWS A5.17-97 includes EM 13 K, EH 11K, EH 12K, EM 12K and EM 14K. When used in a submerged arc welding process requiring low alloy electrodes as per AWS A5.23-97, the new flux is used with electrodes ENi 1K, EF2, EF3, ENi 5, EA3K, EB2, EB3 and EM2 as well as other low alloy and mild steel electrodes and similar electrodes as classified under Euronorm and ISO standards. The use of the novel flux in combination with electrodes mentioned above is representative of different welding processes. In all instances, the diffusible hydrogen is less than 4 ml/100 grams. To accomplish the results attributed to the new low cost highly basic flux, the flux includes a special blend of specific ingredients together with common flux ingredients well known for producing a highly basic flux. By using the specific ingredients a H4 highly basic flux is provided with excellent slag removal characteristics.
In accordance with the present invention, there is provided a highly basic granular or particle flux. This flux is used for arc welding especially in the lower portion of a narrow, deep groove in a thick plate, such as a plate with a thickness of generally over about 0.5 inches. This flux produces less than 7 ml/100 grams of diffusible hydrogen in the weld metal. In accordance with the invention, this novel flux is classified as H4 and comprises a carbon dioxide containing compound with an effective amount of heat releasable carbon dioxide in the range of 0.5-3.5% by weight of the flux. The preferable range is 0.9-1.5% by weight. In this manner, the heat during the arc welding process releases a high volume of carbon dioxide for migration into a surrounding atmosphere. The carbon dioxide is also used for agitating the molten weld metal to assist in the removal of diffusible hydrogen and to obtain an excellent smooth appearance of the resulting bead. The flux also includes over 15% by weight of a low melting point compound and a liquid binder to form the cakes that are ground into the resulting granular flux. The low melting point compounds drastically improve slag removal after the welding operation and cooperates with the high release of carbon dioxide to produce a clean weld bead with an excellent appearance. In accordance with an aspect of the invention, the highly basic flux, as defined above, includes low melting point compounds which are in the form of a silicate or a fluoride. Indeed, the preferred low melting point compound is a silicate, such as sodium, potassium and manganese silicate or combinations of such silicates.
As can be seen, the invention involves a granular highly basic flux that releases a high volume of carbon dioxide and also includes, in combination, a relatively large volume of low melting point material that assist with the releasability of the resulting slag. The slag is agitated by the upwardly and outwardly moving carbon dioxide providing a protective atmosphere around the slag formation.
The flux, as defined above, can be modified to have a particular particle size so that at least 90% of the particles are in the range of a 10-100 mesh screen size. This grading of the particles provides an apparent density of the flux after it has been manufactured whereby the flux pile over the molten metal of the weld bead has a controlled balance between covering the bead and allowing upward movement of gas from the bead and from the flux forming the slag over the bead. To produce this apparent density, or packing density, of the deposited flux pile, the grading provides a broad particle size where over 70% of the particles are in the range of the 10-60 mesh screen size.
The primary object of the present invention is the provision of a low cost highly basic flux for submerged arc welding, which flux is classified as an H4 flux and comprises a carbon dioxide containing compound with an effective amount of heat releasable carbon dioxide to be released during the welding operation to form a carbon dioxide atmosphere around the weld metal and to agitate the molten weld metal and melted slag.
Still a further object of the present invention is the provision of a highly basic, low cost flux for submerged arc welding that produces a low amount of diffusible hydrogen and a slag which is easily removed, especially for deep, narrow groove welding operations.
Yet another object of the present invention is the provision of a flux for submerged arc welding, which flux produces a low diffusible hydrogen level and has excellent slag removal for deep groove welding operations. The resulting weld metal also has excellent weld metal toughness and low cold cracking characteristics.
Yet another object of the present invention is the provision of a flux, as defined above, which flux has an apparent density that covers the molten metal but allows movement of gas upwardly through the flux as it is being melted for forming slag.
Still a further object of the present invention is the provision of a flux, as defined above, which flux provides superior deep groove slag removal, can be used in multiple pass mild steel weldments requiring low temperature impact toughness and in multiple pass low alloy steel weldments requiring low temperature impact toughness. Furthermore, the resulting weld metal formed by submerged arc welding can be used in stress relieved applications.
These and other objects and advantages will become apparent from the following description taken together with the accompanying drawings.